Third Conditional

THIRD CONDITIONAL - Without possibility. Meaning, we are talking about impossible situations because they are in the past and we cannot change what has happened.


These kinds of sentences consist of an 'IF' clause (e.g. If I had gone there...) and the main clause (e.g. he would have seen me).


Here are other examples:
If I had studied harder, I would have got better grades.
If I had had time, I would have gone to the mall with him. 
If we had caught the earlier bus, we would have got there on time.


Notice that the main clause contains 'would', 'could' or 'might.


If I had seen him at the meeting, I would have asked him. (But he wasn't there so I didn't.)
If I had seen him at the meeting, I could have asked him. (But he wasn't there so it wasn't possible.)
If I had seen him at the meeting, I might have asked him. (But I'm not sure. Perhaps if the opportunity had arisen.)




COMMON MISTAKES


1. If I would have known about the practice, I would have gone to it.
Although commonly used in American English, this phrase is grammatically INCORRECT.


Why?
The correct form should be like this:
If + had + past participle, would + have + past participle


If I had known about the practice, I would have gone to it. 
This is CORRECT




2. If I would have asked them, they would have helped me. 
INCORRECT


Why?
In the if-clause we use the past perfect (had + past participle).


If I had asked them, they would have helped me
CORRECT




3. If you had spoken to my friend, she would tell you where I was.
INCORRECT


Why?
The main clause has must be like this:
would + have + past participle.


If you had spoken to my friend, she would have told you where I was.
CORRECT